1. Field of the Invention
The present invention generally relates to a document retrieval system, and more particularly to a document retrieval system in which documents retrieved are given in an order starting from the document closest to the user's requirement so that it is possible to flexibly and quickly retrieve documents.
2. Related Art
In a conventional document retrieval system, at the time of registering documents in the system, an operator selects keywords fitting each document, then, each document for which keywords have been selected is classified on the basis of the thesaurus. When the keywords are added or eliminated, the thesaurus must be renewed. The renewal operation of the thesaurus is quite complex. In addition, when documents which nearly (not exactly) fulfill the user's requirement are retrieved, it is necessary to examine the requirement in detail. To eliminate these disadvantages, the document retrieval system in which a keyword connection and a relevance value are used has been proposed. The keyword connection represents the degree of relationship between each two keywords. The relevance value represents a document's a degree of relevance as a document corresponding to the user's requirement. The greater relevance value is given to the document which is closer to the user's requirement. For example, copending U.S. application Ser. No. 296,769, now U.S. Pat. No. 5,168,565 issued Dec. 1, 1992 proposes a document retrieval system in which the keyword connection and the relevance value are used so that it is possible to flexibly retrieve documents which are required by the user. In addition, Japanease Patent Application No. 63-187988 proposes a document retrieval system having a learning function. In this system, the weight of each relationship in the keyword connection is changed in accordance with the user's judgement and designation so that the user's judgement is reflected in the next retrieval result.
In the proposed document retrieval system, it is possible to retrieve documents only in a case where the user's query is represented by a keyword formula in which keywords are connected by only logical OR. However, a keyword formula which represents the user's arbitrary query generally contains keywords connected by logical AND, OR and NOT. A document retrieval process in which documents are retrieved on the basis of the keyword formula including logical AND, OR and NOT has not yet been developed. That is, in the document retrieval system in which the keyword connection and the relevance value are used, it is impossible to retrieve the document "not having a keyword X", and to retrieve the document "having both keywords X and Y".